High electron mobility transistor manufacturing method and high electron mobility transistor

ABSTRACT

Examples of a high electron mobility transistor manufacturing method includes forming a buffer layer including a nitride semiconductor doped with any one of carbon, iron, and magnesium on a substrate, forming a Schottky layer on the buffer layer, and irradiating the Schottky layer and the buffer layer with electrons or protons.

BACKGROUND

Field

The present invention relates to a high electron mobility transistormanufacturing method and a high electron mobility transistormanufactured by the manufacturing method.

Background Art

Semiconductor elements formed by using nitride semiconductors arepromising high-voltage or high-speed elements because of inherentcharacteristics of compound semiconductor materials. In recent years,high electron mobility transistors (HEMTs) formed by using nitridesemiconductors have been gradually brought into practical use.

In conventional high electron mobility transistors formed by using GaNcompound semiconductors, which are nitride semiconductors, a lowerbuffer layer of AlN or GaN is formed on a sapphire, Si, or SiC substrateat a low temperature, and a buffer layer of GaN and an electron supplylayer of AlGaN are stacked thereon in this order, thus forming aheterojunction structure. A source electrode, a gate electrode, and adrain electrode are provided on the electron supply layer.

In such a high electron mobility transistor, two-dimensional electrongas formed directly under the heterojunction interface between thebuffer layer and the electron supply layer is used as carriers. Whenbias voltages are applied to the source electrode and the drainelectrode, electrons move through the two-dimensional electron gas layerat high speed to travel from the source electrode to the drainelectrode. At this time, the current between the source electrode andthe drain electrode can be controlled by controlling the voltage appliedto the gate electrode to change the thickness of a depletion layerdirectly under the gate electrode.

To reduce a leakage current in the butler layer, the resistance of thebuffer layer needs to be increased. In the case where the resistance ofthe buffer layer is not increased, a drain leakage increases which flowsbetween the source and the drain through the buffer layer in a state inwhich the drain current is turned off. Japanese Patent ApplicationPublication Nos. 2002-57158, 2003-197643, and 2007-251144 proposemethods for increasing the resistance of a buffer layer. In the methodsof Japanese Patent Application Publication Nos. 2002-57158, 2003-197643,and 2007-251144, a buffer layer made of GaN is doped with impuritiessuch as Zn, Mg, or carbon to increase the resistance of the bufferlayer.

The resistance of a semiconductor can also be increased by radioactiveray irradiation such as electron-beam or proton irradiation. JapanesePatent Application Publication No. S50-126180 and International PatentApplication Publication No. WO2012/053081 disclose methods forincreasing the resistance of a semiconductor by irradiating thesemiconductor with an electron beam or protons.

To reduce the drain leakage, impurities need to be added to the bufferlayer. However, if too many impurities are added to the buffer layer,turn-on characteristics are changed. For example, in high electronmobility transistors using GaN semiconductors, if too many impuritiesare added to a GaN semiconductor, traps formed by the impurities causefluctuations in characteristics such as current collapse or thresholdshifts. Current collapse is a phenomenon in which temporal changes in anoutput current lead to poor reproducibility of output currentcharacteristics. It is inferred that such a phenomenon arises asfollows: when a current is passed through a semiconductor element, someof added impurities are charged, and an electric charge built directlyor indirectly influences electron motion in a two-dimensional electrongas layer.

Accordingly, it is difficult to achieve both of a reduction influctuations in turn-on characteristics and a reduction in drainleakage. High electron mobility transistors have been required in whichdrain leakage can be reduced by doping a butler layer with impuritieswhile fluctuations in turn-on characteristics are reduced.

SUMMARY

The present invention has been accomplished to solve the above-describedproblem, and an object of the present invention is to provide a highelectron mobility transistor manufacturing method which can reduce drainleakage by doping a buffer layer with impurities while reducingfluctuations in turn-on characteristics and a high electron mobilitytransistor manufactured by the manufacturing method.

In some examples, a high electron mobility transistor manufacturingmethod includes forming a buffer layer including a nitride semiconductordoped with any one of carbon, iron, and magnesium, forming a Schottkylayer on the buffer layer, and irradiating the Schottky layer and thebuffer layer with electrons or protons.

In some examples, a high electron mobility transistor includes a bufferlayer including a nitride semiconductor doped with any one of carbon,iron, and magnesium, a Schottky layer formed on the buffer layer, a gateelectrode provided on the Schottky layer, a source electrode provided onthe Schottky layer, and a drain electrode provided on the Schottkylayer, wherein first trapping centers originating from the any one ofcarbon, iron, and magnesium and second trapping centers are formed inthe buffer layer, a density of the second trapping centers increasingtoward a top of the buffer layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a high electron mobility transistoraccording to Embodiment 1;

FIG. 2 is a flowchart showing a high electron mobility transistormanufacturing method according to Embodiment 1;

FIG. 3 is a view showing trapping centers;

FIG. 4 is a view showing the relationship between the carbonconcentration and current collapse;

FIG. 5 is a view showing the relationship between the carbonconcentration and drain leakage current;

FIG. 6 is a view showing normal IV characteristics; and

FIG. 7 is a view showing IV characteristics of a high electron mobilitytransistor.

DETAILED DESCRIPTION

High electron mobility transistor manufacturing methods and highelectron mobility transistors according to embodiments of the presentinvention will be described with reference to the drawings. The same orcorresponding components will be denoted by the same reference signs,and the repetition of explanation thereof may be omitted.

Embodiment 1.

FIG. 1 is a cross-sectional view of a high electron mobility transistoraccording to Embodiment 1. This high electron mobility transistorincludes a substrate 1 made of, for example, sapphire, Si, or SiC. Aplurality of compound semiconductor layers are stacked on the substrate1. Specifically, a lower buffer layer 2 formed on the substrate 1, abuffer layer 3 formed on the lower buffer layer 2, and a Schottky layer5 formed on the buffer layer 3 are formed.

The lower buffer layer 2 includes AlN or GaN formed at a lowtemperature, and therefore may be referred to as a low-temperaturebuffer layer. The buffer layer 3 is formed of GaN doped with carbon. TheSchottky layer 5 is formed of AlGaN. The lower buffer layer 2, thebuffer layer 3, and the Schottky layer 5 are stacked in this order onthe substrate 1 to form a heterojunction structure.

A gate electrode 7, a source electrode 6, and a drain electrode 8 areformed on the Schottky layer 5. The source electrode 6 and the drainelectrode 8 as ohmic electrodes are formed by stacking AlTi and Au inthis order on the Schottky layer 5. The gate electrode 7 as a Schottkyelectrode is formed by stacking Pt and Au in this order on the Schottkylayer 5.

In the above-described high electron mobility transistor,two-dimensional electron gas 4 is formed directly under theheterojunction interface between the Schottky layer 5 and the bufferlayer 3. This two-dimensional electron gas 4 is used as a carriertransit layer. Specifically, when bias voltages are applied to thesource electrode 6 and the drain electrode 8, electrons supplied fromthe Schottky layer 5 to the buffer layer 3 move through thetwo-dimensional electron gas 4 to travel to the drain electrode 8. Atthis time, the current flowing from the source electrode 6 to the drainelectrode 8 is controlled by controlling the voltage applied to the gateelectrode 7 to change the thickness of a depletion layer directly underthe gate electrode 7.

FIG. 2 is a flowchart showing a high electron mobility transistormanufacturing method according to Embodiment 1. Steps S1, S2, and S3 arethe steps of stacking nitride semiconductors by MOCVD (Metal OrganicChemical Vapor Deposition). Steps S1, S2, and S3 are performed while100% concentration hydrogen is continuously being fed as carrier gasinto an MOCVD system in which the substrate 1 is disposed, in the stepsof forming the respective layers, with the pressure kept at 26.7 Kpa(200 Torr).

In this environment, in step S1, trimethylaluminum (TMA) and ammonia(NH₃) as raw materials for a compound semiconductor are introduced intothe MOCVD system at flow rates of 150 μmol/min and 11 slm, respectively,and the lower buffer layer 2 of AlN is epitaxially grown on thesubstrate 1 at a growth temperature of 950° C. The layer thickness ofthe lower buffer layer 2 is, for example, 30 nm.

Subsequently, in step S2, trimethylgallium (TMG) and NH₃ are introducedinto the MOCVD system at flow rates of 220 μmol/min and 11 slm,respectively, and the buffer layer 3 of GaN doped with carbon isepitaxially grown on the lower buffer layer 2 at a growth temperature of1050° C. The layer thickness of the buffer layer 3 is, for example, 2μm. The step of forming the buffer layer 3 is referred to as a bufferlayer formation step. In the buffer layer formation step, the growthrate of the buffer layer 3 is controlled so that the concentration ofcarbon added to the buffer layer 3 may be not less than 1×10¹⁶ cm⁻³ normore than 5×10¹⁶ cm⁻³. The conductivity type of the buffer layer 3 is ptype.

Subsequently, in step S3, TMA; TMG, and TMG, and NH₃ are introduced intothe MOCVD system at flow rates of 100 μmol/min, and 1 slm, respectively,and the Schottky layer 5 of AlGaN is epitaxially grown at a growthtemperature of 1050° C. The layer thickness of the Schottky layer 5 is,for example, 30 nm. The step of forming the Schottky layer 5 on thebuffer layer 3 is referred to as a Schottky layer formation step.

Subsequently, step S4 is performed. In step S4, the substrate in whichnitride semiconductors have been epitaxially grown in steps S1, S2, andS3 is irradiated with an electron beam by an electron beam accelerator.This step is referred to as an irradiation step. In the irradiationstep, electrons are applied at an energy of not less than 150 KeV with afluence of 1×10¹⁶ to 1×10¹⁹/cm². In the irradiation step, an electronbeam is applied from above the Schottky layer 5 to irradiate theSchottky layer 5 and the buffer layer 3 with electrons.

FIG. 3 is a view showing trapping centers in the buffer layer 3 and theSchottky layer 5. In the buffer layer 3, first trapping centers 10originating from the doped carbon are formed. The density of the firsttrapping centers 10 is preferably uniform. In this specification, theterm uniform means not just being strictly uniform but also beingsubstantially uniform. In the buffer layer 3 and the Schottky layer 5,second trapping centers 12 originating from defects formed by electronsapplied in the irradiation step are formed. Accordingly, the bufferlayer 3 has the second trapping centers 12 as well as the first trappingcenters 10. The density of the second trapping centers 12 in the bufferlayer 3 and the Schottky layer 5 increases toward the top. Accordingly,the density of the second trapping centers 12 in the buffer layer 3 ishigher in a region near the Schottky layer 5 than in a region near thelower buffer layer 2.

In the irradiation step, electrons may be applied after a mask of heavymetal such as tungsten or zirconium is formed by patterning usingphotolithography. This allows the electron beam to be absorbed by themask, and enables the electron beam to be applied only to a desiredregion. In regions such as a region directly under the drain, chargesand traps formed by electron beam irradiation may influence a change inthe depletion layer. Accordingly, it is preferable that a regiondirectly under the drain electrode 8 is prevented from being irradiatedwith electrons by forming a mask. It should be noted that since nitridesemiconductors have good radiation resistance, even a mask having a lowradiation shielding selectivity can be expected to significantly absorban electron beam.

Subsequently, step S5 is performed. In step S5, first, a mask of asilicon oxide film is formed on the Schottky layer 5 by patterning usingphotolithography. After that, opening portions corresponding to therespective electrode shapes are formed in regions of the mask in whichthe source electrode 6 and the drain electrode 8 are to be formed. Then,Al, Ti, and Au are vapor-deposited in this order in the opening portionsto form the source electrode 6 and the drain electrode 8.

Further, a portion of the mask on the Schottky layer 5 is removed once,and a mask of a silicon oxide film is formed on the Schottky layer 5.After that, an opening portion corresponding to the gate electrode shapeis formed in a region of the mask in which the gate electrode 7 is to heformed. Then, Pt and Au are vapor-deposited in this order in thisopening portion to form the gate electrode 7. Thus, the high electronmobility transistor in FIG. 1 is completed. The resistivity of thebuffer layer 3 of the completed high electron mobility transistor ispreferably not less than 900 Ωcm.

(Current Collapse)

FIG. 4 is a view showing the relationship between the carbonconcentration in the buffer layer 3 and current collapse. The thicknessof the buffer layer 3 is 2 μm. Current collapse is a phenomenon in whichthe on-state resistance value of a high electron mobility transistorduring high-voltage operation becomes higher than the on-stateresistance thereof during low-voltage operation. In other words, theon-state resistance changes according to the state of application of thedrain voltage. To quantify changes in the on-state resistance caused bycurrent collapse, the voltage between the source electrode 6 and thedrain electrode 8 when the high electron mobility transistor is in theon state is scanned in ranges of 0 to 10 V and 0 to 30 V, and the ratiobetween current values obtained for 10 V in the respective ranges isregarded as current collapse. As the current collapse approaches “1.0,”the reproducibility of output current characteristics of the highelectron mobility transistor becomes better. Accordingly, the value ofcurrent collapse is desirably close to 1. The value of current collapseis preferably not less than 0.8, and more preferably not less than 0.9.

FIG. 4 is a view in which actual measurement values are plotted. In FIG.4, the result denoted by “without electron beam irradiation” relates toa high electron mobility transistor manufactured with the irradiationstep skipped. Other three results relate to high electron mobilitytransistors irradiated with electron beams of 1×1e16/cm², 1×1e17/cm²,and 1×5e19/cm² in the irradiation step. FIG. 4 shows that as the carbonconcentration of the buffer layer 3 increases, current collapse becomesworse. In the case of “without electron beam irradiation,” if the carbonconcentration increases to 1×10¹⁷ cm⁻³ or more, current collapsedecreases to 0.9 or less. Meanwhile, it can be seen that in the casewhere an electron beam is applied in the irradiation step, currentcollapse becomes worse as the fluence thereof increases.

(Drain Leakage)

FIG. 5 is a view showing the relationship between the carbonconcentration of the buffer layer 3 and drain leakage current. Drainleakage is a leakage current flowing between the source and the drainwhen the voltage between the source and the drain is 200 V when a highelectron mobility transistor is in the off state. When the drain leakagebecomes large, the element cannot be sufficiently turned off when thehigh electron mobility transistor is in the off state. When the drainleakage exceeds 1×10⁻³ A/m², application as a transistor becomesdifficult. FIG. 5 shows that as the carbon concentration in the bufferlayer 3 decreases, the drain leakage increases. In particular, when thecarbon concentration decreases to a value of not more than 5×10¹⁶ cm⁻³,the drain leakage increases to a value of not less than 1×10⁻³ A/m².

In FIG. 5, the result denoted by “without electron beam irradiation”relates to a high electron mobility transistor manufactured with theirradiation step skipped. Other two results relate to high electronmobility transistors irradiated with electron beams of 1×1e16/cm² and1×1e17/cm² in the irradiation step. FIG. 5 shows that in the case wherean electron beam is applied in the irradiation step, the drain leakagebecomes lower as the fluence thereof increases.

(Nonlinearization of IV Characteristics)

FIG. 6 is a view showing normal IV characteristics of a high electronmobility transistor using a GaN layer. An ideal IV characteristicswaveform of a high electron mobility transistor is linear as shown inFIG. 6. However, if the concentration of carbon added to the bufferlayer becomes too high, the IV characteristics waveform becomesnonlinear. FIG. 7 is a view showing IV characteristics of a highelectron mobility transistor having a buffer layer with a high carbonconcentration. If the carbon concentration of the buffer layer becomestoo high, there occurs a phenomenon in which the drain current does notlinearly increase with respect to the drain voltage. Specifically, ifthe carbon concentration of the buffer layer becomes larger than 5×10¹⁶cm³¹ ³, the IV characteristics waveform becomes nonlinear. Accordingly,in Embodiment 1 of the present invention, the concentration of carbonadded to the buffer layer 3 is set to a value of not less than 1×10¹⁶cm⁻³ nor more than 5×10¹⁶ cm⁻³. Thus, IV characteristics deteriorationcan be avoided.

Referring to the result of “without electron beam irradiation” in FIG. 4and the result of “without electron beam irradiation” in FIG. 5, toobtain a current collapse of not less than 0.9 and a drain leakage ofnot more than 1×10⁻³ A/m² in the case where an electron beam is notapplied, the carbon concentration of the buffer layer 3 is preferablymore than 5×10¹⁶ cm ³ and less than 1×10¹⁷ cm⁻³, and more preferablymore than 7×10¹⁶ cm⁻³ and less than 8×10¹⁶ cm⁻³. It is not easy toaccurately add carbon to a concentration in that range, it is moredifficult to add carbon to a concentration in the above-described rangeover the entire wafer surface.

Moreover, to prevent the nonlinearization of IV characteristics, theconcentration of carbon added to the buffer layer 3 must be not morethan 5×10¹⁶ cm⁻³. Accordingly, in the case where the carbonconcentration is more than 5×10¹⁶ cm ⁻³ and less than 1×10¹⁷ cm⁻³, or inthe case where the carbon concentration is more than 7×10¹⁶ cm⁻³ andless than 8×10¹⁶ cm⁻³, the nonlinearization of IV characteristics cannotbe prevented. Accordingly, without electron irradiation, there is nocarbon concentration range in which the current collapse is not lessthan 0.9, the drain leakage is not more than 1×10⁻³ A/m², and thenonlinearization of IV characteristics can be prevented.

Accordingly, in the high electron mobility transistor according toEmbodiment 1 of the present invention, together with the addition ofcarbon to the buffer layer 3, defects are introduced into the bufferlayer 3 by applying an electron beam so that the current collapse may benot less than 0.9, the drain leakage may be not more than 1×10⁻³ A/m²,and the nonlinearization of IV characteristics may be prevented.Specifically, the concentration of carbon added to the buffer layer 3 isnot less than 1×10¹⁶ cm⁻³ nor more than 5×10¹⁶ cm⁻³. Further, in theirradiation step, electrons are applied at an energy of not less than150 KeV with a fluence of 1×10¹⁶ to 1×10¹⁹/cm².

From FIG. 4, it can be seen that in the case where the carbonconcentration of the buffer layer 3 is not less than 1×10¹⁶ cm⁻³ normore than 5×10¹⁶ cm⁻³, electron irradiation with a fluence of 1×10¹⁶ to1×10¹⁹/cm² can increase the current collapse to 0.9 or more, i.e., canlimit a reduction in the current value caused by a current collapsephenomenon to 10% or less. When the fluence of the electron beam exceeds1×10¹⁹/cm², the current collapse starts significantly decreasing.Accordingly, an upper limit for the fluence is set to 1×10¹⁹/cm².

From FIG. 5, it can be seen that in the case where the carbonconcentration of the buffer layer 3 is not less than 1×10¹⁶ cm⁻³ normore than 5×10¹⁶ cm⁻³, electron irradiation with a fluence of 1×10¹⁶ to1×10¹⁹/cm² can reduce the drain leakage current to 1×10⁻³ A/m² or less.In the case where electron irradiation is performed, lower drain leakagecan be achieved compared to the case where electron irradiation is notperformed. Accordingly, even if the carbon concentration of the bufferlayer 3 is reduced to 3×10¹⁶ cm⁻³ or less, the drain leakage can bereduced to approximately 1×10⁻³ A/m² or less. If the carbonconcentration of the buffer layer 3 is approximately 1×10¹⁶ cm⁻³, thedrain leakage reduction effect of electron beam irradiation is small.Accordingly, the buffer layer needs to be not just irradiated with anelectron beam but also doped with carbon. The concentration of carbonadded to the buffer layer 3 is preferably not less than 1×10¹⁶ cm⁻³ normore than 5×10¹⁶ cm⁻³. If the concentration of carbon added to thebuffer layer 3 is not less than 1×10¹⁶ cm⁻³ nor more than 3×10¹⁶ cm⁻³,the drain leakage can be reduced while the current collapse can be madeclose to 1.

Further, the concentration of carbon added being not less than 1×10¹⁶cm⁻³ nor more than 5×10¹⁶ cm ⁻³ can prevent the nonlinearization of IVcharacteristics. Accordingly, the high electron mobility transistoraccording to Embodiment 1 of the present invention has favorablecharacteristics as a high-frequency amplifier transistor used atfrequencies in or beyond the L band ranging from 0.5 to 1.5 GHz.

Thus, applying an electron beam in addition to doping the buffer layer 3with carbon can increase the resistance of the buffer layer 3 and reducethe drain leakage without greatly deteriorating current collapse.Further, the nonlinearization of IV characteristics can be prevented.Thus, the following trade-off problem can be solved: if the carbonconcentration of the buffer layer is increased to reduce the drainleakage, current collapse and IV characteristics become worse; and, ifthe carbon concentration of the buffer layer is reduced to improvecurrent collapse and IV characteristics, the drain leakage increases.

The high electron mobility transistor manufacturing method and the highelectron mobility transistor according to Embodiment 1 of the presentinvention can be variously modified within a range in which featuresthereof are not lost. For example, the lower buffer layer 2, the bufferlayer 3, and the Schottky layer 5 can be formed of nitridesemiconductors. Moreover, the layer thickness of each layer can bechanged.

These modifications can be applied to high electron mobility transistormanufacturing methods and high electron mobility transistors accordingto embodiments below. It should be noted that the high electron mobilitytransistor manufacturing methods and the high electron mobilitytransistors according to the embodiments below have many things incommon with those of Embodiment 1, and therefore differences fromEmbodiment 1 will be mainly described.

Embodiment 2.

The buffer layer 3 of Embodiment 2 is made of GaN doped with ironinstead of carbon. The added iron allows the first trapping centers 10in FIG. 3 to be introduced. The concentration of iron added to thebuffer layer 3 according to Embodiment 2 is preferably not less than1×10¹⁶ cm³¹ ³ nor more than 5×10¹⁶ cm⁻³, and more preferably not lessthan 1×10¹⁶ cm⁻³ nor more than 3×10¹⁶ cm⁻³. In the buffer layerformation step, the buffer layer is doped with iron at a concentrationof not less than 1×10¹⁶ cm⁻³ nor more than 5×10¹⁶ cm⁻³ or aconcentration of not less than 1×10¹⁶ cm ⁻³ nor more than 3×10¹⁶ cm⁻³.By doping the buffer layer 3 with iron as described above and applyingan electron beam, the same effects as those of the high electronmobility transistor of Embodiment 1 can be obtained.

Embodiment 3.

The buffer layer 3 of Embodiment 3 is made of GaN doped with magnesiuminstead of carbon. The added magnesium allows the first trapping centers10 in FIG. 3 to be introduced. The concentration of magnesium added tothe buffer layer 3 according to Embodiment 3 is preferably not less than2×10¹⁶ cm⁻³ nor more than 2×10¹⁷ cm⁻³, and more preferably not less than2×10¹⁶ cm⁻³ nor more than 1×10¹⁷ cm⁻³. In the buffer layer formationstep, the buffer layer is doped with magnesium at a concentration of notless than 2×10¹⁶ cm⁻³ nor more than 2×10¹⁷ cm⁻³ or a concentration ofnot less than 2×10¹⁶ cm⁻³ nor more than 1×10¹⁷ cm⁻³. By doping thebuffer layer 3 with magnesium as described above and applying anelectron beam, the same effects as those of the high electron mobilitytransistor of Embodiment 1 can be obtained.

A dopant for introducing the first trapping centers 10 is carbon inEmbodiment 1, iron in Embodiment 2, and magnesium in Embodiment 3. Thebuffer layer 3 may be doped with two or more materials selected fromcarbon, iron, and magnesium.

Embodiment 4.

In the irradiation step of the high electron mobility transistormanufacturing method according to Embodiment 4, the Schottky layer 5 andthe buffer layer 3 are irradiated with protons instead of electrons.Specifically, protons are applied at an accelerating voltagecorresponding to an energy of not less than 1 MeV with a fluence of notless than 1×10¹⁵/cm². In Embodiment 4, by applying protons instead ofelectrons, the second trapping centers 12 are introduced. Thus, the sameeffects as those of the high electron mobility transistor of Embodiment1 can be obtained.

In accordance with the present invention, a buffer layer is doped withimpurities and, further, irradiated with an electron beam or protons.Accordingly, drain leakage can be reduced by doping the buffer layerwith impurities while fluctuations in turn-on characteristics arereduced.

What is claimed is:
 1. A high electron mobility transistor manufacturingmethod comprising: forming a buffer layer including a nitridesemiconductor doped with any one of carbon, iron, and magnesium; forminga Schottky layer on the buffer layer; forming a mask on the Schottkylayer; and irradiating the Schottky layer and the buffer layer withelectrons or protons, such that the mask prevents an area of theSchottky layer and the buffer layer under a drain electrode from beingirradiated during the irradiating.
 2. The high electron mobilitytransistor manufacturing method according to claim 1, wherein in thebuffer layer formation, the buffer layer is doped with carbon, and aconcentration of the carbon added to the buffer layer is not less than1×10¹⁶ cm⁻³ nor more than 5×10¹⁶ cm⁻³.
 3. The high electron mobilitytransistor manufacturing method according to claim 1, wherein in thebuffer layer formation, the buffer layer is doped with carbon, and aconcentration of the carbon added to the buffer layer is not less than1×10¹⁶ cm⁻³ nor more than 3×10¹⁶ cm⁻³.
 4. The high electron mobilitytransistor manufacturing method according to claim 1, wherein in thebuffer layer formation, the buffer layer is doped with iron, and aconcentration of the iron added to the buffer layer is not less than1×10¹⁶ cm⁻³ nor more than 5×10¹⁶ cm⁻³.
 5. The high electron mobilitytransistor manufacturing method according to claim 1, wherein in thebuffer layer formation, the buffer layer is doped with magnesium, and aconcentration of the magnesium added to the buffer layer is not lessthan 2×10¹⁶ cm⁻³ nor more than 2×10¹⁷ cm⁻³.
 6. The high electronmobility transistor manufacturing method according to claim 1, whereinin the irradiation, electrons are applied at an energy of not less than150 KeV with a fluence of 1×10¹⁶ to 1×10¹⁹/cm².
 7. The high electronmobility transistor manufacturing method according to claim 1, whereinin the irradiation, protons are applied at an energy of not less than 1MeV with a fluence of not less than 1×10¹⁵/cm².
 8. The high electronmobility transistor manufacturing method according to claim 1, whereinthe buffer layer has a resistivity of not less than 900 Ωcm.
 9. The highelectron mobility transistor manufacturing method according to claim 1,wherein a conductivity type of the buffer layer is p type.
 10. The highelectron mobility transistor manufacturing method according to claim 1,wherein in the irradiation, electrons or protons are applied after themask, which is a mask of heavy metal, is formed.
 11. The high electronmobility transistor manufacturing method according to claim 1, furthercomprising: forming a drain electrode and a source electrode on theSchottky layer after performing the irradiating.